Who wouldn’t enjoy seeing a rainbow? Those curvaceous bow-like seven colors in the sky are beautiful enough to grab everybody’s attention. But have you ever spared a thought on how the rainbow occurs or what makes it look so colorful?

How does the rainbow occur? 

Every time we see a rainbow, it excites us, we take photos and share it, but there are very few people who are curious to find reasons behind whatever processes they see happening around them like the formation of a rainbow.

We are glad you are off of those.  So, what makes a rainbow? Let’s find out.

How does the rainbow occur?
Image: pexels.com

In order to know the facts and the process of the occurrence of the rainbow, we must have some basic knowledge of light.

The light that we see is composed of seven colors namely Red, Orange, Yellow, Green, Blue, Indigo, and Violet. When light travels making some angle with the surface through other mediums like water, glass, etc. which are relatively thicker than air, different colors travel at different speeds relatively slower than air (Red is fastest and Violet is slowest), a phenomenon called refraction of light.

This results in the splitting of light into different colors, a phenomenon called dispersion of light.

A rainbow is just a dispersion of light due to refraction and reflection of the dispersed light towards us.

Formation of the rainbow and interesting facts about it

A rainbow is formed when light coming from the Sun gets dispersed through the surface of a water droplet and gets reflected from the inner surface as shown in the figure below.

The need for water droplet in the atmosphere for the formation of a rainbow cause a rainbow to form generally after the rain with the Sun shining bright.

How Does the Rainbow occur?
Image: possiblywrong.wordpress.com

This means the Sun and the observer should be on the same side. So, to see a rainbow, the Sun has to be behind you! You can see the rainbow and the Sun on the two opposite sides from yourself.

So a morning rainbow has to be in the western sky whereas an afternoon rainbow has to be in the eastern sky.

A rainbow does not have a particular origin on the ground. It is just an optical phenomenon. If we see it above a person then the person sees it differently, as another rainbow.

Since the rainbow depends upon the refraction of light from the water droplets and the water droplets are not stationary, we see the colors dispersed from different droplets at each instant.

Also, if there are two observers viewing a rainbow, they are seeing light dispersed from different droplets i.e different rainbows. Hence, a rainbow is completely observer-dependent.

A double rainbow may be seen if there is a double reflection inside the surface of the water. It is of less intensity and has a reverse color pattern i.e red lies at the bottom and violet at the top.

But what makes a rainbow like a bow?

The rainbow is only seen when the light being dispersed inside the water drop can reflect back. The light is only reflected if the light striking the inner surface of the drop ranges from 0 to 42 degrees, otherwise it is not.

Since 42 degrees is the turning point for light, the light is intense and is clearly visible at an angle close to 42 degrees.

So, our position to see the rainbow must be well defined i.e. we must be able to see such reflecting rays making angles close to 42 degrees. If all the locations of water droplets for which the angle is close to 42 degrees with respect to the observer are taken into consideration, they define a circle.

With the observer included, the circle of the rainbow and the observer defines a cone. But only half of the circle lies above the horizon and hence we see only the semi-circular bow-like structure of the rainbow. However, a full circular rainbow can be seen from airplanes.

This is how a rainbow occurs. It is an observer-dependent optical illusion caused due to dispersion and reflection of light from atmospheric water droplets.

Ashwin Khadka is a PhD Scholar in Solar Cell and Aerosol Laboratory in Korea University, Republic of Korea under Korean Government Scholarship Program. He has a Masters Degree in Physics from Tribhuvan University, Kathmandu, Nepal. He is a science enthusiast, researcher and writer.